Assuming Berkshire's stake is still around that size, Friday's slide cut the value of those shares by $1.168 billion to $12.94 billion.

That's on paper, of course. Berkshire won't actually lose any real money until it sells the shares, and given Buffett's buy-and-hold track record that won't happen until years from now, if it happens at all.

Still, it's a big number and it represents roughly half of Berkshire's paper profits on the stock it started buying in 2011.

While we don't know exactly how much Berkshire paid for IBM, its quarterly SEC filings of U.S. stock holdings do tell us roughly when the stock was purchased.

Using the midpoint of IBM's stock prices at the beginning and end of each quarter and the number of shares purchased in each period, we estimate Buffett's cost basis is around $173 per share. (The large majority of shares were purchased in the second and third quarters of 2011 when the stocks was trading in that vicinity.) That works out to a purchase price of $11.753 billion for the stake.

So, after the slide Berkshire's paper profit is around $1.2 billion, an overall gain of just over 10 percent. Not bad, but just 24 hours ago that paper profit totaled more than $2.3 billion.

Is Buffett concerned? Probably not. As he's said many times, it doesn't bother him when the price of a McDonald's hamburger goes down. He's happy to buy one of his favorite foods at the lower price.

The same is true, he says, when the price tag of a stock he likes goes down. While we won't get any clues until Berkshire's Q2 portfolio report is released in mid-August, it's certainly possible Buffett was buying more IBM on Friday at its new "on sale" price.